Vapor fed liquid-metal cathode



Nov. 3, 1970 w. o. ECKHARDT 3,538,375

VAPOR FED LIQUID-METAL CATHODE Filed April 11, 1968 Wilfried O.Eckhordt, INVENTOR.

vALLEN A. DICKE, Jr.,

AGENT.

United States Patent 3,538,375 VAPOR FED LIQUID-METAL CATHODE Wilfried0.. Eckhardt, Malibu, Calif., assignor to Hughes Aircraft Company,Culver City, Calif., a corporation of Delaware Filed Apr. 11, 1968, Ser.No. 720,694 Int. Cl. H01j 1/12 U.S. Cl. 313-346 11 Claims ABSTRACT OFTHE DISCLOSURE A liquid-metal arc cathode is fed with metal vapor. Thecathode has a wall which forms a transient condensation surface.Temperatures and pressures are maintained so that transient condensationoccurs on these surfaces to permit an arc to run upon the transientlycondensed liquid metal.

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law, 85-568 (72 Stat.435; 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION This invention is directed to the field ofarc cathodes, and especially liquid-metal arc cathodes wherein the metalprovides electrons (from an are spot) for are current.

The prior art liquid-metal arc cathodes primarily are directed to poolcathodes wherein the arc strikes on the pool surface or at the juncturebetween the pool and a wall which confines the pool. When the pool islarge, it is gravitationally retained between the walls and thus suchstructures are not useful in non-gravitational environments, andspecifically such large pool cathodes cannot be used for spacethrusters. For non-gravitational use, a small pool cathode which isforce fed and has a small enough pool so that it can be-retained byadhesive and cohesive forces, is satisfactory.

All pool cathodes which employ a liquid-metal juncture with apool-defining wall have a linear pattern of arc spot activity becausethe are spot is restrained to act at the juncture of the pool with thepool-keeping wall. This results in a high thermal input density to thenarrow line representing the wall and pool juncture, with consequenthigh temperatures and high atom evaporation rate from the adjacentliquid-metal pool surface. In those cases Where it is desirable tomaintain the electron to atom emission ratio as high as is practical,this high local thermal input density is undesirable. This localizationis obviated when no specific liquid-to-wall line'is provided, buttransient vapor condensation provides a larger area upon which the arespot operate. Such operation provides lower average thermal inputdensity to minimize evaporation.

SUMMARY In order to conveniently understand this invention, it can bestated in essentially summary form that it is directed to a vapor fedliquid-metal cathode. The vapor fed liquid-metal cathode is operated atsuch temperature and pressure that transient condensation occurs fromthe vapor being fed to the cathode, which condensation is positioned forarcing activity.

Accordingly, it is an object of this invention to provide a liquid-metalcathode which has a wall therein on which liquid-metal vapor cantransiently condense to provide a zone on the wall for are spotactivity. It is a further object of this invention to provide aliquid-metal cathode which is fed with vaporized liquid metal, andwherein a certain amount of the vapor is condensed upon condensationwalls so that are spot activity can take place anywhere there is vaporcondensed upon the condensa- 3,538,375 Patented Nov. 3, 1970 tion walls.It is a further object of this invention to provide a liquid-metalcathode which operates at such temperatures and pressures so that whenliquid-metal vapor is fed to the cathode, transient condensation occursthereon to permit are spot activity in the areas of such transientcondensation. It is a further object of this invention to provide acathode which is fed by vaporized liquid metal so that the cathode isindependent of positioning and is not influenced by gravity direction orthe absence of gravity. It is another object of this invention toprovide a vapor fed liquid-metal cathode which is useful in mercury aredevices, including space thrusters, mercury arc rectifiers, switch tubesand thelike. Other objects and advantages of this invention will becomeapparent from a study of the following portion of the specification, theclaims and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWING The single figure of the drawing isgenerally a section through a cathode suitable for feeding withvaporized liquid-metal, together with the feed structure therefor.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, thecathode is generally indicated at 10 and its feed system is generallyindicated at 12. The cathode 10 is useful in many environments whichrequire an arc cathode. This utility extends from an electron source inan ion thruster, such as is disclosed in H. R. Kaufman Pat. No.3,156,090, to usage in electric current switching and rectifyingdevices. In the case of use as an electron source for ionization in theKaufman thruster, the liquid metal supplied by the feed system alsosatisfies some or all of the need for ionizable material. However, asset forth more completely below, the vapor fed liquidmetal cathode canbe operated at a higher electron to atom emission ratio so that inoperation of such Kaufman thrusters an additional inlet for propellant,beyond that represented by atoms discharged from the face of the cathode10, may be necessary.

Higher thruster efliciency and lower specific thermal loading resultsfrom thermally decoupling the pool-keeping zone from the evaporatorzone, as compared to a small liquid-fed pool cathode, because therequired electron to atom emission ratio may be obtained with less vaporflow construction, and thus less plasma loss, downstream from theelectron emitting zone.

It is possible to achieve such high electron to atom emission ratiosthat only a fraction of the total expellant flow needs to be fed throughthe cathode. This is a consequence of the reduced thermal power inputdensity of the vapor fed cathode. The resulting design freedom for theexpellant injection pattern results in additional thruster efiiciencyimprovements as well as complete interchangeability between the vaporfed liquid-metal cathode and other cathodes, without any thrustermodifications.

In the case of electric switch tubes and rectifiers, the cathode 10supplies the electrons for the arc with a minimum of atomic vaporizationwhich is discharged into the general atmosphere within the tube.

Cathode 10 has a face 14 in which is located a recess which providestransient condensation surface 16. In addition, a plug is insertedinteriorly of the transient condensation surface 16 in order to providea facing transient condensation surface 18. These two surfaces arepreferably curved surfaces of revolution so they provide a divergingnozzle to provide optimum expansion of vaporized liquid metal whichmoves past them. Annular feed channel 20 opens between surfaces 16 and18 so that vapor may be fed adjacent the surfaces. The divergence ofthese surfaces from the throat at the opening of feed channel 20provides maximum velocity of the vapor away from face14, in a directionnormal to face 14. One or more feed tubes 22 are connected from the backof cathode to annular feed channel 20. The abovedescribed constructionis the preferred construction, but ordinary single divergent nozzleconstruction, either conical or curved, is also useful. Additionally,rectangular or slot cathode openings, again preferably divergent, mayfind utility in specific applications.

A convenient way of feeding liquid metal to the cathode 10 is generallyindicated at 12. However, the structure is merely illustrative and themany other equivalent structures can be used instead. Cylinder 24contains liquid mercury or other liquid metal 26 in its bottom. Piston28 is positioned within the cylinder so that when piston 28 is forceddownward, the liquid metal 26 within the cylinder is pressurized. Thepiston 28 can be forced down by any convenient means, and in theillustrated embodiment, gas under pressure is admitted at inlet 30 topressurize the interior of the cylinder above the piston. Flow ismeasured by indicator 32 which reads the position of piston 28 withinthe cylinder. Indicator 32 is a commercially available dial indicatorwhich indicates by the position of its hand with respect to its dial therelative position between the end of its probe and its housing. Sincethe probe of indicator 32 touches piston 28, and the housing ofindicator 32 is mounted upon cylinder 24, the position of its hand onits dial face indicates the position of the piston with respect to thecylinder. By computing these readings with respect to time, the flowrate in outlet tube 34 is readily and accurately determined.

Outlet tube 34 conveys the liquid metal 26 from the interior of cylinder24. Vaporizer screen 36 is positioned within tube 34. The screen is suchthat it is not wet by the liquid metal and thus minisci of the liquidmetal are exposed between the wire meshes of the screen and are directeddownstream of the mesh to the vapor filled portion of th tube downstreamfrom the vaporizer mesh. The relationship between the surfacecharacteristics of a liquid and a solid against which the liquid liesdetermines whether or not the liquid wets the solid. In the case ofmercury against glass, the miniscus formed is convex on the gas side, ascompared to the concave miniscus of water against glass when viewed fromthe gas side. In the present case, the material of the screen is chosenwith respect to the liquid-metal material that the liquid metal does notwet the screen. Thus, the minisci between the screen wires are convex,as viewed from the vapor side of screen 36. Heater 38 is positionedaround tube 34 downstream from the vaporizer mesh, all the way from thevaporizer mesh to the cathode body. Heater 38 acts to supply the heat ofvaporization, to cause the liquid metal to vaporize from the minisci inthe vaporizer mesh, and thus the evaporization rate is controlled by thetemperature of heater 38. Furthermore, heater 38 maintains the vaporfilled portion of the feed tube at above the temperature adjacent thevaporizer mesh so that the liquid-metal vapor is superheated to preventsubstantial condensation upon the tube walls. Furthermore, thetemperature of the cathode 19 is controlled by heat exchange jacket 40which also maintains the temperature in the annular feed channel andfeed tube 22 above the metal condensation temperature. Thus, all of thesurfaces adjacent the vaporized liquid metal are above the temperatureat the vaporizer mesh, in order to prevent condensation of liquid-metaldroplets upon the walls. Thus, the temperature of all the walls facingthe vapor are maintained sufiiciently high to prevent condensation inmulti-atomic thicknesses. However, at least the transient condensationsurfaces 16 and 18 are maintained at a sufficiently low temperature thata partial mono-atomic coverage can be attained.

Metal which is liquid at a reasonable, preferably room, temperature ispreferred, a d While m rcu y is the pr 4 ferred liquid metal for use inthe supply of vaporized liquid metal to the cathode 10, cesium, lithiumandgallium are also examples of suitable materials. Thus, with respectto the exemplary figures given below, mercury is the metal used in theillustration.

When the cathode 10 is placed in an operative environment, and mercuryis the liquid metal, temperatures at the vaporizer mesh typically rangefrom 180 to 250 C. with corresponding pressures of mercury vapor justdownstream of the mesh from 10 torr to torr. Since the pressure in thearc chamber at face 14 is in the order of 1() torr, the pressure drop isprincipally taken up in feed tube 22 and annular feed channel 20. Thisis a convenient way to supply mercury vapor, for while it is possible toplace the vaporizer mesh at the throat immediately below transientcondensation surfaces 16 and 18, in such case the temperature at thevaporizer mesh must be kept very low in order to prevent excessiveevaporation at the arc chamber pressure.

When cathode 10 is placed into a suitable environment, at a reducedpressure such as at 10 torr, together with an anode and a suitablesource of electric current and an igniter, and vapor feed is started,the vapor from vaporizer 36 proceeds up to feed tube 22 to a positionadjacent the transient condensation surfaces 16 and 18. Transientcondensation of the liquid-metal vapor on the walls occurs all along thechannels up to and including the surfaces 16 and 18 to permit anelectric are spot to act thereon. Transient condensation refers to themaintenance of the walls at such a temperature that a partial coveragewith condensed atoms of the liquid metal results. Other factors, inaddition to surface temperature, which affect this coverage are the arcemission current and the metal vapor flow. In any event, when transientcondensation occurs, there is a surface coverage of condensed atoms onthe surface which provides a less than unity coverage of a mono-atomiclayer of the surfaces 16 and 18. The arc acts preferentially as close tothe face 14 as there is adequate liquid-metal fil-m deposition.

A very wide range of temperatures for the cathode transient condensationsurfaces and the vaporizer mesh are possible for proper operation. Aspreviously stated, the transient condensation surfaces must be at ahigher temperature than the vaporizer in order to maintain steady flowconditions. A film of liquid metal can condense upon the transientcondensation surfaces even though they are at higher than equilibriumcondensation temperature at that pressure. The equilibrium condensationtemperature is the temperature at which the atoms condensing from thevapor equals the number of atoms evaporating to the vapor. Theequilibrium condensation temperature is, thus, necessarily lower thanthe transient condensation temperature, because at equilibriumcondensation temperature, condensed atoms can reach unity on thesurface, and can build up into multi-layers and into droplets. In fact,higher than equilibrium temperature is preferred in order to preventdroplets from condensing, for only a thin condensed film is necessaryfor operation in the manner described. In various different applicationsof the cathode 10, different operating constraints would call fordifferent operating conditions. In thrusters, in order to reject areheat, a selected cathode temperature is typically 300 C. Once thecathode temperature is chosen, the vaporizer temperature must be belowthat value, but the exact value of the vaporizer temperature isdetermined by the flow impedance between the vaporizer and cathode face.

Heat exchange jacket 40 controls the temperature of the passages withinthe cathode and of the transient condensation surfaces. At high arecurrent, cooling will be necessary to maintain the transientcondensation surfaces at a proper temperature. However, at low arccurrent, heating may be necessary, depending upon radiation and conduction losses. If the temperature of the transient condensationsurfaces is too high for a given liquid-metal vapor flow rate, transientcondensation does not occur and the arc will not run.

There is no fixed line between regions with adequate liquid-metal filmand the adjacent regions with inadequate liquid-metal film on thetransient condensation surface for are activity. As a result, the arespot does not run in a line as is characteristic of a pool placedadjacent such a surface. Instead, the are spot randomly occurs at thejuncture between such condensed liquid metal and the adjacent wall.Thus, since the transient condensation is random, virtually the entirewall over a period of time is available to define an are spot locationwith transiently condensed liquid metal. Thus, a larger area of wall isactive and heat discharge into the cathode structure is spread over alarger area. This results in reduced local temperatures with higherelectron to atom emission ratios, the ratio extending to at least tenelectrons per atom for mercury when the cathode temperature is 300 C. Inoperating such a cathode with vapor feeding at 300 C., the heat inputinto the cathode from the arc is in the Order of watts of heating powerper ampere of current in the arc. This represents an improvement ascompared to forced liquid-fed small pool cathodes operating at the sametemperature.

When the cathode is used in a Kaufman-type thruster, the higher ratio ofelectrons per atom provides increased ionization ability when operatedat a given temperature, thus providing flexibility in choosing eitherhigher temperatures, together with a lighter cathode, or choosing asecondary inlet for the ionizable material, rather than use only theliquid-metal vapor emitted from the cathode as the ionizable material.

This invention having been described in its preferred embodiment, it isclear that it is susceptible to numerous modifications and embodimentswithin the ability of those skilled in the art and without the exerciseof the inventive faculty.

What is claimed is:

1. A cathode, said cathode being positionable in an arc chamber forsupplying electrons to an electric arc:

said cathode comprising a transient condensation surface on saidcathode;

temperature control means connected to said transient condensationsurface for maintaining the temperature of said transient condensationsurface so that transient condensation of liquid-metal vapor cancontinuously occur on said transient condensation surface;

vaporized liquid-metal feed means connected for continuously directingvaporized liquid metal adjacent said transient condensation surface sothat liquid metal continuously transiently condenses on said transientcondensation surface so that an electric arc can continuously run ontransiently condensed liquid metal.

2. The cathode of claim 1 wherein said cathode has a face and saidtransient condensation surface is angularly positioned with respect tosaid face.

3. The cathode of claim 2 wherein said transient condensation surface isa surface of revolution.

4. The cathode of claim 3 wherein said transient condensation surface isa divergent nozzle.

5. The cathode of claim 3 wherein there are first and second facingtransient condensation surfaces annularly positioned adjacent said faceof said cathode and a feed tube is connected between said liquid-metalvapor feed means and between said first and second transientcondensation surfaces.

6. The cathode of claim 1 wherein said vaporized liquid-metal feed meanscomprises a vaporizer for heating and vaporizing liquid metal.

7. The process for supplying electron emission material to an electricarc comprising the steps of:

providing a transient condensation surface for the transientcondensation thereon of a thin film of liquid metal;

continuously feeding vaporized liquid metal adjacent the transientcondensation surface;

continuously controlling the temperature of the transient condensationsurface for continuously maintaining the transient condensation surfaceat transient condensation temperature; and

continuously transiently condensing liquid metal on the transientcondensation surface.

8. The process of claim 7 wherein said controlling step comprisesmaintaining the transient condensation surface at a temperature abovethe equilibrium condensation temperature at the pressure adjacent thetransient condensation surface.

9. The process of claim 8 wherein said feeding step comprises thefeeding of superheated vaporized liquid metal past the transientcondensation surface.

10. The process of claim 9 wherein the step of feeding superheatedvaporized liquid metal comprises vaporizing liquid metal away from thetransient condensation surface, superheating the vaporized liquid metaland expanding the superheated vapor as it is transported from the pointof vaporization past the transient condensation surface.

11. The process of claim 7 wherein the step of feeding vaporized liquidmetal adjacent the transient condensation surface comprises superheatingthe liquid-metal vapor, expanding the superheated vapor in a divergentnozzle, the surfaces of which comprise the transient condensationsurface.

References Cited UNITED STATES PATENTS 3,370,198 2/1968 Rogers et al315-111 RAYMOND F. HOSSFELD, Primary Examiner U.S. Cl. X.R.

